Carotenoids represent one of the most widely distributed and structurally diverse classes of natural pigments, producing pigment colors of light yellow to orange to deep red. Eye-catching examples of carotenogenic tissues include carrots, tomatoes, red peppers, and the petals of daffodils and marigolds. Carotenoids are synthesized by all photosynthetic organisms, as well as some bacteria and fungi. These pigments have important functions in photosynthesis, nutrition, and protection against photooxidative damage. An essential compound, animals and humans must obtain carotenoids through dietary sources, as they do not have the ability to synthesize carotenoids.
Structurally, carotenoids are 40-carbon (C40) terpenoids derived from the isoprene biosynthetic pathway and its five-carbon universal isoprene building block, isopentenyl pyrophosphate (IPPP). This biosynthetic pathway can be divided into two portions: the upper isoprene pathway, which leads to the formation of IPPP, and the lower carotenoid biosynthetic pathway, which converts IPPP into long C30 and C40 carotenogenic compounds.
The genetics of carotenoid pigment biosynthesis has been extremely well studied in the Gram-negative, pigmented bacteria of the genera Pantoea, formerly known as Erwinia. In both E. herbicola EHO-10 (ATCC 39368) and E. uredovora 20D3 (ATCC 19321), the crt genes are clustered in two genetic units, crt Z and crt EXY1B (U.S. Pat. No. 5,656,472; U.S. Pat. No. 5,5545,816; U.S. Pat. No. 5,530,189; U.S. Pat. No. 5,530,188; U.S. Pat. No. 5,429,939). Despite the similarity in operon structure, the DNA sequences of E. uredovora and E. herbicola show no homology by DNA-DNA hybridization (U.S. Pat. No. 5,429,939).
Although more than 600 different carotenoids have been identified in nature, only a few are used industrially for food colors, animal feeding, pharmaceuticals and cosmetics. Presently, most of the carotenoids used for industrial purposes are produced by chemical synthesis; however, these compounds are very difficult to produce chemically. Natural carotenoids can either be obtained by extraction of plant material or by microbial synthesis. At the present time, only a few plants are widely used for commercial carotenoid production. However, the productivity of carotenoid synthesis in these plants is relatively low and the resulting carotenoids are very expensive.
A number of carotenoids have been produced from microbial sources. For example, Lycopene has been produced from genetically engineered E. coli and Candia utilis. β-carotene has been produced from E. coli, Candia utilis and Pfaffia rhodozyma. Zeaxanthin has been produced from recombinant E. coli and Candida utilis, and Astaxanthin has been produced from E. coli and Pfaffia rhodozyma. 
Additionally genes encoding various elements of the carotenoid biosynthetic pathway have been cloned and expressed in various microbes. For example genes encoding lycopene cyclase, geranylgeranyl pyrophosphate synthase, and phytoene dehydrogenase isolated from Erwinia herbicola have been expressed recombinantly in E. coli. Similarly genes encoding the carotenoid products geranylgeranyl pyrophosphate, phytoene, lycopene, b-carotene, and zeaxanthin-diglucoside, isolated from Erwinia uredovora have been expressed in E. coli, Zymomonas mobilis, and Saccharomyces cerevisiae. Similarly, the carotenoid biosynthetic genes crtE, crtB, crtI, crtY, and crtZ isolated from Flavobacterium have been recombinantly expressed.
The methods for producing carotenoids available to date, however, suffer from low yields and reliance on expensive precursors and methods. A method that produces higher yields of carotenoids less expensively is clearly needed.